The resistance against using Thunderbird as the default Ubuntu e-mail client has come down to the program lacking Microsoft Exchange support at this time (Evolution supports it) and calendar/contacts integration. To get Thunderbird well integrated with the Ubuntu desktop it would need evolution-data-server support, calendar integration, and contacts integration. Some of these items have been somewhat achieved while other areas are still being tackled. There's also more opportunities for better desktop integration with its look and feel to provide a consistent user experience.
Work is already under-way on integrating Unity desktop support into Thunderbird along with Ubuntu One contact syncing support, indicator-message integration, and various other items.
No firm decision has yet been made whether Mozilla Thunderbird will become the default e-mail application in Ubuntu 11.10, but the preliminary decision is leaning towards using Thunderbird. If a change doesn't occur with the Oneiric Ocelot, it's likely no change will occur until Ubuntu 12.10 due to 12.04 being a Long-Term Support release where they're less prone to introduce such front-facing changes.
I for one vote for Mozilla Thunderbird as the default e-mail client. The official UDS Budapest notes for the Thunderbird discussion can be found on this page.